This invention relates generally to a miniature motor used for audio equipment, video equipment, precision equipment, etc., and more specifically to a miniature motor on which improvement has been made so that stable motor performance can be achieved by preventing the abnormal wear of the sliding contact surface of the commutator and the generation of polymers caused by sliding contact between the commutator and the brushes.
FIG. 1 is a longitudinal sectional front view of the essential part of a miniature d-c motor to which this invention is applied. In FIG. 1, numeral 1 refers to a case made of a metallic material, such as mild steel, formed into a bottomed hollow tubular shape and having an arc-segment shaped permanent magnet 2 fixedly fitted to the inner circumferential surface thereof. In the case 1 fitted is a rotor 5 comprising an armature 3 facing the permanent magnet 2 and a commutator 4. Numeral 6 refers to a case cap made of an insulating material, such as a synthetic resin, and formed so as to be fitted to an open end of the case 1. Numeral 7 refers to a brush arm adapted so as to make sliding contact with the commutator 4 and fitted to the case cap 6, together with an input terminal 8 electrically connected to the brush arm 7. Numerals 9 and 10 refer to bearings each fixedly fitted to the bottom of the case 1 and the central part of the case cap 6, respectively, to rotatably support a shaft 11 constituting the rotor 5.
With the aforementioned construction, when current is fed from the input terminals 8 to the armature 3 via the brush arms 7, and the commutator 4 constituting the rotor 5, rotating force is imparted to the armature 3 placed in a magnetic field formed by the permanent magnet 2 fixedly fitted to the inner circumferential surface of the case 1, causing the rotor 5 to rotate. This causes external equipment (not shown) to be driven via the shaft 11 on the output side.
In a conventional type of miniature motor having the aforementioned construction, brushes (not shown) provided at the tips of the brush arms 7 and the commutator 4 may be worn out in a short period due mainly to arc discharge or Joule heat generated at the sliding contact area between the brushes and the commutator 4, or the physical contact between the brushes and the commutator 4. This abnormal wear of the brushes and the commutator 4 could deteriorate the electrical connection between the brushes and the commutator 4 and therefore the stability of the initial motor performance, reducing the service life of the motor.
When such a miniature motor is used in an atmosphere containing an organic gas, black-colored insulating polymers are produced on the sliding contact area between the brushes and the commutator 4 owing to the aforementioned arc discharge or Joule heat. These insulating polymers may make the state of contact between the brushes and the commutator 4 unstable, increasing contact resistance. This could also reduce the service life of the motor.
To overcome the above problems, a construction where polyvalent alcohol, etc. is encapsulated in the motor case to produce an atmosphere of the polyvalent alcohol has been proposed (refer to Japanese Published Unexamined Patent Application No. Sho-60(1985)-162449). Because of its small molecular weight, the polyvalent alcohol readily evaporate, making its atmosphere in the motor case short-lived. Thus, the polyvalent alcohol has an insufficient effect in extending motor life.